Re: [PATCH v2 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap

[David Hildenbrand]

On 27.06.25 09:36, Barry Song wrote:
> On Fri, Jun 27, 2025 at 7:15 PM Lance Yang <lance.yang@xxxxxxxxx> wrote:
> > On 2025/6/27 14:55, Barry Song wrote:
> > > On Fri, Jun 27, 2025 at 6:52 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > > > On Fri, Jun 27, 2025 at 6:23 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> > > > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > > > >
> > > > > As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
> > > > > can read past the end of a PTE table if a large folio is mapped starting at
> > > > > the last entry of that table. It would be quite rare in practice, as
> > > > > MADV_FREE typically splits the large folio ;)
> > > > >
> > > > > So let's fix the potential out-of-bounds read by refactoring the logic into
> > > > > a new helper, folio_unmap_pte_batch().
> > > > >
> > > > > The new helper now correctly calculates the safe number of pages to scan by
> > > > > limiting the operation to the boundaries of the current VMA and the PTE
> > > > > table.
> > > > >
> > > > > In addition, the "all-or-nothing" batching restriction is removed to
> > > > > support partial batches. The reference counting is also cleaned up to use
> > > > > folio_put_refs().
> > > > >
> > > > > [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@xxxxxxxxxx
> > > >
> > > > What about ?
> > > >
> > > > As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
> > > > may read past the end of a PTE table when a large folio spans across two PMDs,
> > > > particularly after being remapped with mremap(). This patch fixes the
> > > > potential out-of-bounds access by capping the batch at vm_end and the PMD
> > > > boundary.
> > > >
> > > > It also refactors the logic into a new helper, folio_unmap_pte_batch(),
> > > > which supports batching between 1 and folio_nr_pages. This improves code
> > > > clarity. Note that such cases are rare in practice, as MADV_FREE typically
> > > > splits large folios.
> > >
> > > Sorry, I meant that MADV_FREE typically splits large folios if the specified
> > > range doesn't cover the entire folio.
> >
> > Hmm... I got it wrong as well :( It's the partial coverage that triggers
> > the split.
> >
> > how about this revised version:
> >
> > As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
> > may read past the end of a PTE table when a large folio spans across two
> > PMDs, particularly after being remapped with mremap(). This patch fixes
> > the potential out-of-bounds access by capping the batch at vm_end and the
> > PMD boundary.
> >
> > It also refactors the logic into a new helper, folio_unmap_pte_batch(),
> > which supports batching between 1 and folio_nr_pages. This improves code
> > clarity. Note that such boundary-straddling cases are rare in practice, as
> > MADV_FREE will typically split a large folio if the advice range does not
> > cover the entire folio.
>
> I assume the out-of-bounds access must be fixed, even though it is very
> unlikely. It might occur after a large folio is marked with MADV_FREE and
> then remapped to an unaligned address, potentially crossing two PTE tables.

Right. If it can be triggered from userspace, it doesn't matter how 
likely/common/whatever it is. It must be fixed.

> A batch size between 2 and nr_pages - 1 is practically rare, as we typically
> split large folios when MADV_FREE does not cover the entire folio range.
> Cases where a batch of size 2 or nr_pages - 1 occurs may only happen if a
> large folio is partially unmapped after being marked MADV_FREE, which is
> quite an unusual pattern in userspace.

I think the point is rather "Simplify the code by not special-casing for 
completely mapped folios, there is no real reason why we cannot batch 
ranges that don't cover the complete folio.".

--
Cheers,

David / dhildenb